Battle for Middle Earth 2 - Good Campaign - Mission 1 - Rivendell

If you go straight to the House of Elrond and fight along the way, you will probably lose every Lorien Warrior in the process, and possibly have a hard time keeping Glorfindel alive too.
Instead, proceed step by step and scout your way there. There are scripted events that don't really become active until you are near and you can therefore stop and let your troops recover if they have Veterancy or if you have the Heal power.
Turn the Lorien Warriors on the Guard stance to increase their armor and survivability. Any time you see Lorien Warriors, try to turn that on as soon as possible.
As you head east, you will encounter Lorien Warriors fighting with Goblin Warriors. As soon as you can, set their Stance to Guard to increase their chances of survival while you attack with your own forces. Do NOT retreat them -- the game calculates advantages for flank and rear attacks, so if you tell them to run away, they will definitely be slaughtered.

Furtehr east is an encounter is with goblins cutting down a Mallorn Tree. It's too late to save the tree (the event is scripted so that although the goblins keep attacking the tree while you spy it from a distance, the tree is destroyed shortly after the event is fully activated when you get closer) and if you charge in blindly you will get tangled up in a pretty big fight and could well lose all your units. There's no one to save here, so you can take time to be cautious
Instead, pull back and scout a bit with Glorfindel to find the extent of their forces and pick them off from the rear, starting with the archers. Since elves can hide in trees and you can force them to stay hidden with the Guard stance, you can position them a bit south behind their last rank (archers) and charge in with Glorfindel first. By the time you finish them off, the Goblin Warriors may have turned their attention to you, but this is better than facing all the goblin battalions at once; if not, pull back and wait for your battalions to recover and restore their numbers. Move up to finish the last of the goblins, then head for the bridge.
Once Glorfindel is past level 1, he can handle whole battalions of goblins on his own, especially with his Blade of Purity power, which when activated lets him hit more than one unit at a time.

At the bridge into Rivendell proper, you can get two more battalions of Lorien Warriors if you can save them. Head for the gate across the bridge, but DO NOT move Glorfindel into the light (marking a quest location) yet. The map is now fully revealed to you. Check out what is where in Rivendell first and send your elves to a Mirror of Galadriel. Rivendell isn't yet set to your ally or your holdings, so they won't heal yet. After the scripted dialogue with Elrond, your elves will have been moved to a statue around the middle of Rivendell. Move them back to the Mirror to heal.

The rest of the mission is pretty straightforward. Don't build too many Lorien Warriors to start because you might then hit the Command Point limit and not be able to build archers when they become available.
The second wave beelines for the nearest barracks, and the giants and trolls can trample. Sometimes a troll in the second wave might be in the fog of war and your forces won't engage. The second wave is not finished and the event scripts won't advance. Explore the western pass to reveal any remaining enemies and clear them.
Watch for Elrond and Arwen to be available as heroes and add them to your hero line. Glorfindel and Gloin can level up to 3, while Arwen and Elrond can level up to 5.

The Wyrm's fire breath can set units ablaze, which does some damage over time and can ultimately kill them. If you have a Builder nearby, they can extinguish flames.